A session border gateway (SBG) can be logically split into two entities, namely a signaling path function and a data path function. This split provides clarity when trying to narrow the functions addressed by different SBG types. These two functions may co-reside within the same physical element or may be separated with a protocol acting as the interface between them.
Traditionally, SBG features and functions are implemented in different ways:                a) Stand-alone devices called session border controllers (SBCs);        b) Integrated in the network infrastructure elements such as routers, gateways, switches, optical line termination (OLT) equipment, and Internet protocol based Digital Subscriber Line Access Multiplexer (IP-DSLAM); or        c) A device at the border of wireless access and wireline core transport network.        
A list of SBG features and functions can be found in, for example, MSF Session Border Gateway Requirements specification that is available at http://www.msforum.org/techinfo/approved/MSF-PS-SBG-001.00-FINAL.pdf. Moreover, an example of an infrastructure element-based implementation of SBG features and functions can be found in U.S. Pat. No. 7,656,797 entitled, “Method, Computer Program Product, And Apparatus For Providing A Distributed Router Architecture.”
Certain drawbacks of utilizing infrastructure element-based implementation of SBG features and functions include:                a) Costs;        b) Time required for testing and integration with network;        c) Static allocation of resources;        d) Less flexibility in repositioning the resources; and        e) Tighter coupling of computing and communications resources with pre-designed border features and functions.        
Service providers in a dynamic and continuously-evolving networking and service development environment need:                a) Protection of investment, i.e., investment in the resources that can be rapidly repurposed for different revenue generating applications and services; and/or        b) Agility and flexibility, i.e., deploying emerging features and functions utilizing the computing and communications resources that already exist in the network.        